Characteristics and origin of the gas and condensate in the Yinggehai Basin, offshore South China Sea: evidence for effects of overpressure on petroleum generation and migration

Three superimposed pressure systems have developed in the Yinggehai Basin. The strongly overpressured rocks have abnormally low vitrinite reflectance (Ro) and Rock-Eval Tmax values and display abnormal C15+ hydrocarbon distributions. Such organic maturity anomalies were confirmed to be the result of overpressure retardation. Commercial gas fields with carbon dioxide content up to 93% and gas shows with nitrogen content more than 50% have been found in the basin. Methane- and carbon dioxide-dominated gases have low C2+ hydrocarbon contents (less than 4%) and heavy methane δ13C and δD values, and the associated condensates have high diamondoid indices and very low C25+ saturated hydrocarbon concentration. At the same time, the methane- and carbon dioxide-dominated gases have high branched and cyclic light hydrocarbon contents, and associated condensates have abnormally low heptane and isoheptane values, low sterane isomerization values and abnormally high concentrations of isoprenoid hydrocarbons relative to adjacent n-paraffins. These distinct characteristics seem to result from the effects of overpressure on both hydrocarbon generation and migration.